Diatom-mediated food web functioning under ocean artificial upwelling

Enhancing ocean productivity by artificial upwelling is evaluated as a nature-based solution for food security and climate change mitigation. Fish production is intended through diatom-based plankton food webs as these are assumed to be short and efficient. However, our findings from mesocosm experiments on artificial upwelling in the oligotrophic ocean disagree with this classical food web model. Here, diatoms did not reduce trophic length and instead impaired the transfer of primary production to crustacean grazers and small pelagic fish. The diatom-driven decrease in trophic efficiency was likely mediated by changes in nutritional value for the copepod grazers. Whilst diatoms benefitted the availability of essential fatty acids, they also caused unfavorable elemental compositions via high carbon-to-nitrogen ratios (i.e. low protein content) to which the grazers were unable to adapt. This nutritional imbalance for grazers was most pronounced in systems optimized for CO2 uptake through carbon-to-nitrogen ratios well beyond Redfield. A simultaneous enhancement of fisheries production and carbon sequestration via artificial upwelling may thus be difficult to achieve given their opposing stoichiometric constraints. Our study suggest that food quality can be more critical than quantity to maximize food web productivity during shorter-term fertilization of the oligotrophic ocean.


Figure S1
: Fish of the nutrient composition experiment.a) Natural prey size range of the study fish.Individuals that had been feeding on the wild plankton community in the harbor were analyzed for stomach content.The longest dimension of the prey body (prosome for copepods) was assessed under a stereomicroscope.For each fish individual, the smallest and largest prey found and the average size across all prey are shown and employed in linear regressions (± 95% confidence ranges).The size reference of the fish used for the experiments inside the mesocosms is provided in orange.Our fish overlap strongly in prey size range, and thus trophic function, with both larvae and adults of commercially important small pelagic fishes such as sardines and anchovies [1][2][3][4].b) Size of fish individuals of the larvae class after 6 days inside the mesocosms under Si:N manipulation.Averages across several individuals (# in grey) were employed in linear regressions.Abundances were highly variable, assumable due to random mortality of this more sensitive life stage, and unrelated to Si:N (regression: R 2 = 0.25, p = 0.206).including all metazoan zooplankton averaged across all mesocosms for each size class.Note that the high Oikopleura counts on day 9 come from a short and randomly occurring bloom of these organisms in a single mesocosm.The copepods comprise mainly herbivorous and omnivorous taxa that are common in the oligotrophic surface ocean [5] and important prey for small pelagic fishes and fish larvae of other fisheries species [6,1,2,7,3,4].b) Trophic level estimate for different grazers, expressed as the enrichment in d 15 N in copepods compared to the phytoplankton baseline.Averages across several samples (# in grey) were employed in linear regressions (± 95% confidence ranges).Table S2: Linear models to establish that, integrated over the experimental period of the upwelling intensity experiment, upwelling mode was only minor driver of diatoms (a) and fatty acids (b).Intensity (continuous), mode (categorical) and their interaction were employed as explanatory variables (type III test).Table S4: Linear mixed models for the relationship between zooplankton and particulate organic matter stoichiometry.Both experiments were included with one data point per mesocosms and sampling day.Particulate organic C:N was employed as continuous fixed effect and mesocosm (n=18) as random effect (random intercept, restricted maximum likelihood fit, Satterthwaite approximation, [8,9]).

Figure S2 :
Figure S2: Zooplankton during the nutrient composition experiment.a) Abundance-based community composition including all metazoan zooplankton averaged across all mesocosms for each size class.Note that the high Oikopleura counts on day 9 come from a short and randomly occurring bloom of these organisms in a single mesocosm.The copepods comprise mainly herbivorous and omnivorous taxa that are common in the oligotrophic surface ocean[5] and important prey for small pelagic fishes and fish larvae of other fisheries species[6,1,2,7,3,4]. b) Trophic level estimate for different grazers, expressed as the enrichment in d 15 N in copepods compared to the phytoplankton baseline.Averages across several samples (# in grey) were employed in linear regressions (± 95% confidence ranges).

Figure S3 :
Figure S3: Food web base during the nutrient composition experiment.Shown are temporal developments and phase averages employed in regressions (95% confidence ranges via dashed lines).

Figure S4 :
Figure S4: Food web base during the upwelling intensity experiment.a) Full illustration of 'diatom dominance' as the explanatory variable used in several further analyses.b-e) Particulate organic matter fatty acids.Shown are temporal developments and phase averages employed in regressions (95% confidence ranges via dashed lines).

Figure S5 :
Figure S5: Zooplankton during the upwelling intensity experiment.a) Abundance-based community composition including all metazoan zooplankton averaged across all mesocosms for each size class.Note that while Oikopleura has high abundances, its body is partly gelatinous with low organic matter content and its long tail makes it collect in larger size classes.In terms of biomass, the composition pattern would thus shift in favor of the copepods.b) Fatty acidbased estimate on the change in the the contribution of diatoms to the diet of copepods.All samples of the two size classes are combined for figure2b.c) Trophic level estimate for grazers expressed as the enrichment in d 15 N in copepods compared to the phytoplankton baseline: across groups per day (left) and across days per group (right).The three groups were combined for figure 2c.Averages across several samples (# in grey) were employed in linear regressions (± 95% confidence ranges).

Figure S6 :
Figure S6: Relationship between P and N content in particulate organic matter during artificial upwelling.All sampling days of both experiments are included as individual data points and employed in regressions (95% confidence ranges via dashed lines).Three data points with extremely high C:P ratios were seen as technical errors and excluded.
squares; dfNum and dfDen = numerator and denominator degrees of freedom

Table S1 : Linear regressions of the nutrient composition experiment with
Si:N as explanatory variable to accompany fig 1.

Table S3 : Linear regressions for the upwelling intensity experiment
to accompany figure 2.